The present invention relates to rate-responsive pacing methods and systems, and more particularly to a rate-responsive pacing method or system wherein the minimum oxygen saturation level of the venous blood in the right atrium is used as a control parameter to adjust the rate at which electrical stimulation pulses are delivered to a patient's heart.
A pacemaker is a medical device, usually an implantable medical device, that provides electrical stimulation pulses to a patient's heart at a controlled rate for the purpose of controlling the heart rate. Most modern implantable pacemakers can be programmed to operate in several modes, as required by the needs of a particular patient. Several common modes of operation provide stimulation pulses only when the patient's heart does not beat by itself at a minimum rate. In such mode(s), the stimulation pulses are provided only when needed, or "on demand", thereby preserving the limited power source of the implanted pacemaker for the longest possible time. Typically, the manner in which such demand pacemakers operate is to define a basic pacing interval (sometimes referred to as an "escape interval") and to wait and see if the heart beats during this interval. (A heart beat is determined by sensing a "P-Wave" indicating contraction of the atria, or an "R-wave", indicating contraction of the ventricles.) If so, the basic pacing interval starts over, and no stimulation pulse is provided. If not, a stimulation pulse is provided at the end of the pacing interval. In this manner, the pacemaker's pacing interval defines the rate at which stimulation pulses are provided to the heart in the absence of naturally occurring heart beats. It is noted that pacemakers may be employed that stimulate either, or both, chambers of the heart (i.e., either the right atrium and/or the right ventricle).
A rate-responsive pacemaker is a pacemaker that automatically adjusts the pacing interval, or the rate at which stimulation pulses are provided to the patient's heart, as a function of the sensed physiological needs of the patient. That is, every person has times when his or her heart needs to beat fast, and times when his or her heart should beat slow. For example, physical activity causes a person's heart rate to increase in order to compensate for the increased oxygen demands of the muscle tissue undergoing the physical activity. Similarly, physical inactivity, such as prolonged periods of sleep or rest, allow a person's heart rate to decrease because the oxygen demands of the body tissue are less. A rate-responsive pacemaker thus attempts to sense the physiological needs of a patient at a particular time, e.g., by sensing physical activity or inactivity, and adjusts the pacing interval of the pacemaker accordingly.
The operation and design of pacemakers, including rate-responsive pacemakers, are known in the art. See, e.g., Furman, et al., A Practice of Cardiac Pacing, (Futura Publishing Co., Mt. Kisco, N.Y. 1986); Moses, et al., A Practical Guide to Cardiac Pacing (Little, Brown & Co., Boston/Toronto 1983); U.S. Pat. No. 4,712,555 (Thornander et al); U.S. Pat. No. 4,856,523 (Sholder et al). U.S. Pat. No. 4,712,555 (Thornander et al.) is a particularly comprehensive reference explaining the general operation of a rate-responsive pacemaker, and the application of one particular type of physiological parameter (a timing interval) for controlling such pacemaker. U.S. Pat. No. 4,712,555 is incorporated herein by reference. Further, it is known in the art to sense several different physiological parameters as the control parameter of a rate-responsive pacemaker. One common type of sensor is an activity sensor that senses the physical activity level of the patient. See, e.g., U.S. Pat. No. 4,140,132, issued to Dahl; and U.S. Pat. No. 4,485,813, issued to Anderson.
Other types of sensors used in prior art rate-responsive pacers include sensors that sense respiration rate, blood and/or body temperature, blood pressure, the length of the Q-T interval, and the length of the P-R interval.
Of particular significance to the present invention, it is also known in the art to use an implantable sensor to determine the oxygen content of blood and to use such sensor in a rate-responsive pacemaker. See, e.g., U.S. Pat. Nos. 4,202,339; 4,399,820; and 4,815,469. Further, recent studies have suggested that mixed venous oxygen saturation provides one of the best indications available of physiological need, especially for low and medium levels of exercise (physical activity). It has thus been suggested that mixed venous oxygen saturation, when combined with other parameters, provides a very useful control parameter for controlling a rate-responsive pacemaker. See, Stangl, et al., "A New Multisensor Pacing System Using Stroke Volume, Respiratory Rate, Mixed Venous Oxygen Saturation, and Temperature, Right Atrial Pressure, Right Ventricular Pressure and dP/dt," PACE, Vol. 11, pp 712-724 (June 1988).
Unfortunately, while oxygen saturation may be one of the most sensitive parameters to indicate low and medium level exercise, the techniques heretofore used in the prior art to sense oxygen saturation have masked out the most beneficial information provided by this parameter. For example, oxygen saturation is typically sensed optically using a sensor that includes both a source of light, such as a light emitting diode (LED), and a means for detecting light, such as a phototransistor. The sensor, including both LED and phototransistor, is positioned in an appropriate location to sense venous oxygen saturation, e.g., in the right atrium. Light energy is directed to the blood in the right atrium from the light source. The amount of light energy reflected back to the phototransistor is a function of the properties of the blood, including the level of oxygen saturation of the blood. Thus, by monitoring the ratio of emitted light energy to reflected light energy, it is possible to measure the blood oxygen saturation level of the blood in the right atrium. However, because the return blood in the right atrium comes from all parts of the body, it contains significantly different levels of blood oxygen saturation, reflecting the different activity levels of various parts of the body. That is, if the patient is walking, the blood returned from the legs and arms (assuming the arms are swinging as the legs are walking) will have a significantly lower oxygen content than will blood from other parts of the body. This is because the leg and arm muscle tissue is working harder (and therefore consuming more oxygen) than is muscle tissue at other body locations.
Hence, the blood oxygen saturation measured in the right atrium tends to fluctuate over a wide range, depending upon how thoroughly the blood is mixed at the time the measurement is made. To compensate for these fluctuations, the prior art teaches averaging or integrating the measurement over a sufficiently long period of time to smooth out such fluctuations. Disadvantageously, such averaging or integrating masks out the most beneficial portions of the measurement--the oxygen saturation level of the blood returned from the arms and legs, or other parts of the body that are experiencing physical activity. What is needed, therefore, is a technique or method for measuring the oxygen saturation of the blood returned from just those portions of the body undergoing the greatest physical activity, or otherwise isolating that portion of the fluctuating oxygen saturation measurement indicative of such physical activity.
Further, when measuring blood oxygen saturation using an optical sensor that measures reflected light energy, and when such sensor is positioned in the heart, the amount of reflected light energy detected by such sensor is significantly influenced by optical reflections from the heart wall or valves. Such optical reflections disadvantageously give erroneously high readings. Hence, what is needed is a sensing method or system that senses only those optical reflections from returned blood, not from optical reflections occurring within the heart. More particularly, what is needed is a system and method for sensing optical reflections from blood returned to the heart from only those body portions undergoing the most strenuous physical activity.
The present invention advantageously provides a system and method of blood oxygen saturation measurement that addresses the above and other needs.